Functional Ecology 2005 19, 594–601 © 2005 British Ecological Society 594 Blackwell Publishing, Ltd. Diversity-dependent productivity in semi-natural grasslands following climate perturbations A. KAHMEN,*† J. PERNER‡ and N. BUCHMANN*§ *Max Planck Institute for Biogeochemistry, Hans-Knöll-Straße 10, D-07745 Jena, ‡Institute of Ecology, University of Jena, Dornburger Straße 159, D-07743 Jena, Germany Summary 1. The consequences of globally declining biodiversity and climate change for ecosystem functions are intensively debated topics in ecological research. However, few studies have investigated potential interactions, or the combined effects of both scenarios, for ecosystem functioning. In the work presented here we tested the hypothesis that increasing plant diversity acts as insurance for ecosystem functions during extreme weather events which are predicted by climate change scenarios. 2. We measured the effect of plant diversity on above- and below-ground productivity in semi-natural grasslands following experimentally induced early summer drought. To test the insurance hypothesis directly, we determined in each community the range of δ 13 C values of individual plant species as drought stress indicators. 3. Increasing plant diversity significantly enhanced below-ground productivity as a consequence of simulated drought, while above-ground productivity was reduced independently of plant diversity. 4. Plants shifting carbon allocation to below-ground compartments during drought maintain various aspects of ecosystem services and functions. Although we were not able to detect physiological evidence for the insurance hypothesis, we conclude from our below-ground results that plant diversity is an essential entity of ecosystems for maintaining ecosystem functions in a changing climate. Key-words: below-ground productivity, biodiversity, climate change, disturbance, drought, ecosystem functions, insurance hypothesis, stable isotopes, PCA Functional Ecology (2005) 19, 594–601 doi: 10.1111/j.1365 -2435.2005.01001.x Introduction There is a long-standing debate in ecological research about the relationship between biodiversity and stab- ility (MacArthur 1955; McNaughton 1977; Leps, Osbornovakosinova & Rejmanek 1982; Pimm 1984). The focus of the debate has shifted in recent years from population and food-web levels to the ecosystem level, where the influence of biodiversity on the stability of ecosystem functions is of interest. This is partly a consequence of the dramatic worldwide loss in spe- cies diversity (Chapin et al . 1997), and the predicted increase in extreme climatic events such as flood, frost, fire, storm and drought as a result of global climate change (IPCC 2001). Climate change scenarios, as well as the causes and consequences of decreasing diversity, are beginning to be well understood (Loreau et al . 2001). However, it remains unknown if ecosystems with high diversity can buffer the effects of climate change and if, consequently, ecosystems reduced in diversity are more sensitive to climate change. If so, climate change effects on ecosystem functions will become more severe if diversity is continuously lost. According to the so-called ‘insurance hypothesis’, species diversity influences the stability or resistance of ecosystem functions against environmental pertur- bations (McNaughton 1977; Doak et al . 1998; Yachi & Loreau 1999). The hypothesis is based on the assumption that different species react differently to environmental change. With increasing species diversity, the range of species with different responses to environmental change will therefore also increase in an ecosystem. Consequently, a more diverse ecosystem has a higher likelihood of containing species that are adapted to a changed environment and can compensate for the decline of less adapted species, thus maintaining stable ecosystem functions. †Author to whom correspondence should be addressed. E- mail: akahmen@bgc-jena.mpg.de §Present address: Institute of Plant Sciences, ETH Zürich, Universitätsstraße 2, CH-8092 Zürich, Switzerland.